There are instances where solutions contain many additives only several of which are objectionable but which cannot be removed by ion exchange because of the expense of rejuvenating the ion exchange resin. As an example, consider the tertiary treatment of sewage. After sewage water is subjected to primary and secondary treatment, the organic substances have been pretty well broken up and bacteria have been substantially eliminated by processes such as chlorination but there are still a number of substances which are objectionable. Usually ammonium salts result from the degradation of the organic components and phosphates are present from detergents which have been added to the sewage by residential and commercial facilities.
These components are objectionable because ammonium salts and phosphates encourage the growth of algae. When the concentration of algae becomes excessive in the water into which the sewage is dumped, fish are killed because when algae die and decompose they use up much of the oxygen from the water.
In addition to the ammonium salts and phosphates found in sewage water, calcium, magnesium, sodium, nitrates, sulphates and chloride are also present in varying amounts. The presence of these substances is acceptable but the standard ion exchange technique removes them along with the major pollutants, that is, ammonia and phosphates and thus they block out resin capacity. In order to regenerate the resin, it is therefore necessary to remove the calcium, magnesium, sodium, sulphates, nitrates and chloride as well as the phosphates and ammonia. Thus in order to regenerate the resin, it is necessary to treat it with an excessive amount of regenerant. This adds considerable cost to the process.